In article ,
says...
why don't you go with a piece of Thomson shaft, it is already hard and comes
undersize by a couple of thousandths.

Reid Tool carries it so does MSC

It comes in 36" length

Reid part # TLM_1605 $42.48

msc part #56540495 $25.30


Or $17.46 from McMaster. The size is closer to .0005/.001
under nominal, and it's very straight.

Exact cut lengths can be special ordered thru a bearing
distributor if you don't want to fuss with cutting through
the hard case, but it can be cut easily with carbide or
with abrasives.

Ned Simmons